Heretofore, there has been proposed a structure for a front portion of a vehicle body, where a flange which is an edge of an inner panel with a groove shape is layered with a flange of an outer panel, these are joined together to form a closed cross-section shape, these flanges are bent from the edge to the middle in the width of the front side frame, and extended at the middle in the longitudinal direction of the front side frame. This front side frame is capable of absorbing shock according to a predetermined deformation pattern in a sure manner (e.g., see Japanese Unexamined Patent Application Publication No. 6-135355).
There has also been proposed a structure where, of the layered and joined flanges, the tip flanges where the load is input are bent in a crank shape, so the tip flanges do not peel away from each other but rather deform in a wave-like shape, and this wave-like shape is propagated throughout the remaining flanges, whereby energy absorption at the time of collision can be improved (e.g., see Japanese Unexamined Patent Application Publication No. 2005-199741).
Further, there has been proposed a structure where a bulkhead is provided within the front side frame at a position matching a joint with a cross member, whereby the front side frame behind the position where the bulkhead has been provided can be allowed to compress and deform in a stable manner (e.g., see Japanese Unexamined Patent Application Publication No. 7-149255).
Through diligent study, the inventors found the following problems. With the related art according to Japanese Unexamined Patent Application Publication No. 6-135355, upon a load being input due to the front of the vehicle colliding with an object, it is conceivable that bending will first start from the portion where the flanges have been bent, so the bending positions are limited, and there is little freedom in design. With the arrangements disclosed in Japanese Unexamined Patent Application Publication Nos. 2005-199741 and 7-149255, the front side frame compresses and deforms in wave-like shapes in order, so there is little freedom in design.
For example, there is an arrangement for a front side frame where, upon a shock load being input from the front, the front side frame is bent at three positions, thereby absorbing the shock. With the front side frame, there is the need to form tire clearance portions to increase the steering angle of the front wheels, and the need to curve the font side frame so as to mount an engine with an east-west (transverse) layout, and so forth. This means that the shape of the front side frame is not uniform but bent, so causing bending load to occur at the predetermined bend positions might be achieved by adding reinforcing members near other bending positions so as to set difference in strength, but smoothly bending the three bending positions in order is difficult. Moreover, adding reinforcing members leads in increased weight and costs for the front side frame.